The invention relates to a MAC protocol for OFDMA PONs.
Synchronizing with optical line terminals (OLTs) is a basic requirement of optical network units (ONUs) in time division multiplexing (TDM) passive optical networks (PONs), where ONUs transmit their data traffic over the time slots allocated by OLT so as to avoid data collision. The synchronization need, on one hand, implies implementing a sophisticated clock synchronization system in ONUs. On the other hand, the non-negligible synchronization time may result in extra packet delay, reduced bandwidth utilization, and possibly some unnecessary energy consumption.
In one optical system with OFDMA PON uses OFDMA as its modulation and access scheme. OFDMA PON divides the upstream/downstream bandwidth in baseband into multiple subcarriers with orthogonal frequencies. These subcarriers are dynamically allocated to different ONUs based on their real-time incoming traffic information. As compared to TDM PON and WDM PON, OFDMA PON enjoys numerous advantages such as high speed transmission, fine granularity bandwidth allocation, and color-free ONUs. To eliminate the synchronization need in OFDMA PON, one way is to dedicate subcarriers for each ONU. Then, ONUs can transmit any time they want on their respective dedicated subcarriers. This scheme is similar to the transmission in WDM PONs where each ONU is dedicated with one or more wavelengths. However, no statistical multiplexing gain can be exploited among traffic of ONUs.